Functionalists in philosophy of mind traditionally raise
two major arguments against the type identity theory: (1) psychological
states are
multiply realizable so that there are no one-to-one mappings
of psychological states onto neural states and (2) the most that evidence
could ever establish is the correlation of psychological and neural
states, not their identity. We defend a variant on the traditional type
identity theory which we call heuristic identity theory (HIT) against
both of these objections. Drawing its inspiration from scientific practice,
heuristic identity theory construes identity claims as hypotheses that
guide subsequent inquiry, not as conclusions of the research.

Introduction

Functionalists in philosophy argue that the type identity theory advances
an unjustifiably strong account of the metaphysics of mind. Ironically,
one of the first proponents of using functional criteria to identify mental
states, David Armstrong, viewed functional analysis as a means for supporting
the identity theory. The dominant versions of functionalism, though, reject
the type identity of mental and physical states, since their relations
are many-to-many or at least one-to-many, not one-to-one. This is known
as the multiple realizability objection to the identity theory.

The identity theory faces another objection to the effect that empirical
investigations can never establish anything more than a correlation between
mental events and physical events. We shall call this the correlation
objection. Recent discussions of consciousness (Chalmers, 1996) have pressed
this objection anew. The general argument is that, at best, neurophysiological
approaches isolate brain states that correlate with conscious states. They
cannot justify identifying these neural states with the conscious states
(especially in light of their disparate properties). They can only establish
their correlation.

A richer appreciation of the course of scientific research over time
and of the thoroughly hypothetical character of all identity claims in
science argues for a heuristic conception of the identity theory. Identity
claims typically play a heuristic role in science. Scientists adopt
them as hypotheses in the course of empirical investigation to guide subsequent
inquiry--rather than settling on them merely as the results of such inquiry.
Defending heuristic identity theory (HIT) from both the multiple
realizability and correlation objections, we will argue that mapping at
least some mental states (viz., many that figure in scientific psychology)
one-to-one with physical states is a perfectly normal part of research
in cognitive neuroscience and that the results often provide ample support
for these hypotheses.

HIT versus the multiple realizability
objection: The role of comparative studies

Underlying the multiple realizability objection is the assumption that
looking across species will yield type differences in brains despite the
type identities of mental states (Putnam, 1967). Perusing research in cognitive
neuroscience, though, casts doubts on that assumption.

It seems obvious that when individuals from different species are in
the same mental state, their neural states will differ. After all, even
within the mammalian order, brains from different species clearly look
different. Thus, it may prove surprising to learn that the neurobiological
practice of identifying brain areas and brain processes
(1) is and historically has been a comparative endeavor
(Bechtel & Mundale, 199). To appreciate how a comparative approach
informs neurobiological proposals, consider the examples that follow (two
historical and one contemporary).

The first involves research on mapping the brain into functionally relevant
areas by using cytoarchitectural tools, a project now largely associated
with Korbinian Brodmann, but pursued at the turn of the century by many
others (e.g., Oskar and Cécile Vogt, Constantin von Economo). A
key foundation for Brodmann's demarcation of areas was his demonstration
that cortex generally consists of six layers, for which he reported comparative
studies involving fifty-five species. He distinguished areas on the basis
of the relative thickness of these layers (e.g., layer 4 was very thick
in areas 1, 2, and 3, but much thinner in area 4) and the particular types
of neurons found (e.g., pyramidal cells). In identifying brain areas, Brodmann
worked comparatively; besides his well-known map of the human cortex (figure
1), he generated maps for the lemur, flying fox, rabbit, and others. (See
figure 2 for an example.) For Brodmann, success in finding comparable areas
in different species despite differences in brain shapes and in the relative
location of areas was pivotal in establishing the reality of distinct functionally
relevant areas in the brain. (Brodmann, 1909/1994).

Figure 1: Brodmann's human map

Figure 2: Brodmann's lemur map

Figure 3: Brodmann's hedgehog map

Although Brodmann's goal was to identify functionally relevant brain
areas, neuroanatomical techniques generally do not suffice to do so. Before
the rise of functional brain imaging, neuroscientists primarily relied
on lesion and electrophysiological techniques. David Ferrier (1876) used
electrophysiological techniques in the 1870s, employing mild electrical
stimulation to map brain areas in a large number of species, including
monkey, dog, jackal, cat, rabbit, guinea pig, and rat. He utilized a numbering
scheme shown in Table 1 to record the functional character of the responses
elicited (figure 4). Although he was unable to use this technique on humans,
Ferrier's goal was to extrapolate his results to humans and in his final
chapter he proposed how areas found in other species project onto the human
cortex (figure 5).

1. Opposite hind limb is
advanced as in walking2. Flexion with outward rotation
of the thigh, rotation inwards of the leg, and flexion of the toes3. Movements comparable to
1 and 2, plus movements of the tail4. Opposite arm is adducted,
extended, and retracted, the hand pronated5. Extension forward of the
opposite arm (as if reaching or touching something in front).a,b,c,d Clenching of the fist6. Flexion and supination of
the forearm7. Retraction and elevation
of the angle of the mouth8. Elevation of the ala
of the nose and upper lip9. Opening of the mouth, with
protrusion of the tongue10. Opening of the mouth,
with retraction of the tongue.11. Retraction of the
angle of the mouth.12. Eyes open widely,
pupils dilate, and head and eyes turn to the opposite side.13,13' Eyes move to the opposite
side14. Pricking of the opposite
ear, head and eyes turn to the opposite side, pupils dilate widely.15. Torsion of the lip and
semiclosure of the nostril on the same side

Figure 5: Ferrier's projection of areas on monkey brain (left) from
which he elicited stimulation to the human brain (right).

Historically, neuroscientific practice routinely involved identifying
brain areas and processes across broad range of species as belonging to
the same type. These practices continue. Maps of cortical areas have become
more refined as neuroscientists have developed additional tools, such as
connectivity analysis, to identify brain areas. Still, maps of, for example,
visual processing areas in the brain--developed by Ungerleider and Mishkin
(1982) and van Essen and Gallant (1994)--are based principally on studies
of macaque monkeys. Oddly, when they consider theories of mind-brain relations,
philosophers seem to forget that the overwhelming majority of studies have
been on non-human brains. Experimentally induced lesions and cell recording
are two of the principal tools for unraveling the functional significance
of different areas, but for obvious ethical reasons these are largely restricted
to non-human animals. Although the ultimate objective is to understand
the structure and function of the human brain, neuroscientists depend upon
indirect, comparative procedures to apply the information from studies
with non-human animals to the study of the human brain. For example, they
determine the location of areas such as V4 and MT in the human brain by
using neuroimaging techniques to find where tasks that would drive cells
in these areas in macaques result in increased blood flow in humans (Zeki
et al., 1991).

Why do the differences between the brains of different organisms, which
so exercise philosophers, not impede neurobiological research? Part
of the reason seems to be that neurobiologists often employ criteria for
type identities in brains that are more coarse-grained than most philosophers
have envisaged. Of course, the philosophical practice of comparing mental
states across species is also rather coarse-grained. But no one should
begrudge that. When pondering hypotheses that identify the psychological
structures and processes of minds with the biological structures and processes
of brains, surely one crucial issue is insuring that we compare analyses
with compatible grains. Accordingly, neurobiologists do treat psychological
processes (albeit not those of folk psychology, but ones that figure in
information processing accounts of psychological function) as comparable
across species, but they largely elude the problem of multiple realization
by working with analyses from the two pertinent levels that have at least
roughly similar grains (Bechtel & Mundale, 1999).
(2) Ascertaining the compatibility of "grain" between research
at two different levels is one of the most basic examples of the co-evolution
of sciences.

When Putnam (1967) employed for his example of common psychological
states hunger in humans and octopi, his grain for type identifying psychological
states was not especially fine. Such a broad extension of psychological
types poses problems for the functional identification of psychological
states, since the links to other mental states and to behaviors that are
central to functional analyses differ profoundly between such radically
different species. Still, given that evolution tends to conserve and extend
existing mechanisms rather than create new ones, researchers could well
end up type identifying even the neural mechanisms involved in hunger in
the octopus and human, which would substantially defuse Putnam's intuitively
plausible example. This is not to rule out the possibility of radically
different ways of performing similar functions emerging in evolution. However,
when researchers discover multiple mechanisms for performing similar functions,
such as alternative pathways for processing visual input in invertebrates
and vertebrates, it provides an impetus for psychologists to search for
functional (behavioral) differences that motivate the differentiation of
types at the psychological level as well. Acknowledging the possibility
of different mechanisms performing similar functions does not preclude
maintaining type distinctions that preserve one-to-one mapping between
neural and psychological types. With just such lessons in mind, HIT looks
to the comparative practices of neurobiology to dodge the multiple realizability
objection.

Champions of the correlation objection, i.e., the objection that identity
theorists can never establish the actual identity of neural and psychological
states but only their regular correlation, assume (correctly) that identity
theorists bear the burden of evidence in this debate. In a perversely Humean
spirit, though, they set the bar impossibly high, requiring identity theorists
to establish each identity claim's truth--in effect--beyond a shadow of
a doubt. Discredited in the philosophy of science, verificationism, oddly,
enjoys new life in the philosophy of mind.

Neurobiological practice provides direction for answering this objection
too. Scientists often propose identities during the early stages of their
inquiries. These hypothetical identities are not the conclusions of scientific
research but the premises. They serve as heuristics for guiding scientific
discovery. (McCauley, 1981) Instead of appealing to Leibniz's law of the
identity of indiscernables as a metaphysical principle for settling things
a priori, they opportunistically exploit its converse, the indiscernability
of identicals, to guide subsequent empirical research. This formulation
of Leibniz's law entails that what we learn about an entity or process
under one description must apply to it under its other descriptions. Scientists
propose these identities, in part, precisely because the two accounts do
not mirror one another perfectly. They use each to guide discovery in the
other.

This involves employing what we learn through psychological research
to guide the discovery and elaboration of neural mechanisms and what we
learn about neural mechanisms to develop more sophisticated psychological
models. We will sketch the case of visual processing, which has involved
a set of related hypothetical identities that have linked neural and psychological
investigation for over a hundred years in an on-going story of progressive
theoretical revision at both levels of analysis. Researchers revised their
initial identification of cortical visual processing with processing in
V1 as they recognized, with the help of increasingly sophisticated neurobiological
accounts, that a much larger part of cortex subserves vision; these revisions
in the neurobiological account are now inspiring revisions in the psychological
account of vision. For practitioners in these fields at the end of a century
of research, both the comparable complexity and the general compatibility
of models from the psychological and neural sides of the divide render
philosophers' disquiet about the incompleteness of the evidence a needlessly
fastidious extravagance. Few researchers would contest identifying visual
processing with processing in the areas denoted in the figure of the flattened
cortex of the macaque by van Essen and Gallant (figure 6).

Figure 7: Talbot and Marshall's (1941) mapping of areas in the visual
field onto areas of primary visual cortex through single-cell recording.

Research on the neural mechanisms of vision began in the last half of
the nineteenth century with efforts to locate a visual center in the brain.
Based upon neuroanatomical studies indicating that the optic tract, after
projecting to a part of the thalamus known as the lateral geniculate nucleus
(LGN), subsequently projects to the occipital lobe (Meynert, 1870) and
upon clinical evidence concerning visual deficits following stroke and
other damage to the occipital lobe, most researchers identified it as the
locus of visual processing. Ferrier, however, dissented, arguing on the
basis of lesion studies in monkeys and his stimulation studies that the
angular gyrus in the parietal cortex was the locus of vision. One critical
piece of evidence that suggested that Ferrier was wrong was the discovery
that the organization of the occipital lobe reflected topographical layout
of the visual field. Early evidence for this came from Salomen Henschen's
(1893) attempt to map lesions in the occipital cortex and corresponding
deficits in the visual field in humans, but the map he offered reversed
the mapping contemporary scientists accept. During the Russo-Japanese War
Tatsuji Inouye and during World War I Gordon Holmes and William Tindall
Lister developed the modern account of the topographical arrangement of
occipital cortex from their studies of wounded soldiers (Glickstein, 1988).
Using single cell recording in cat and monkey Talbot and Marshall (1941)
corroborated their proposals (figure 7).

Discovering this organization in the occipital cortex supported the
hypothesis that it was the location for visual processing in the brain,
but it left most of the questions about how the brain processes visual
information unanswered. Stephen Kuffler's (1953) research on the retina
and LGN had revealed the distinctive center-surround response of cells
in those areas (i.e., some cells would respond to a stimulus when it was
in the center of their visual field but be suppressed when it was in their
surround, while others would respond to a stimulus in the surround but
not the center). Two researchers in his laboratory, David Hubel and Torsten
Wiesel, set out to find similar response patterns in the occipital cortex
of cats and monkeys but discovered that cells there were responsive to
bars instead (Hubel & Wiesel, 1962, 1968). What they termed simple
cells responded to bars of specific orientation at specific locations,
while what they termed complex cells responded to bars of specific
orientation at any location in a cell's receptive field and might show
selective responses to bars moving in some particular direction. While
Hubel and Wiesel's demonstration of specific visual function in V1 provided
further support for its identification as a visual area and important details
about the character of visual processing, it also showed that visual processing
could not be identified with V1 alone. They ended their 1968 paper with
a prophetic remark:

Specialized as the cells of 17 are, compared with rods and cones, they
must, nevertheless, still represent a very elementary stage in the handling
of complex forms, occupied as they are with a relatively simple region-by-region
analysis of retinal contours. How this information is used at later stages
in the visual path is far from clear, and represents one of the most tantalizing
problems for the future. (Hubel and Wiesel, 1968, p. 242)

Although Karl Lashley (1950) had strongly resisted proposing specialized
visual processing areas outside of V1, Alan Cowey (1964), relying on single
cell recording, demonstrated that V2 also contained a systematic map of
the topographical organization of the visual field. In 1965 Hubel and Wiesel
(1965) showed that yet a third visual area, V3, preserved the topographical
organization. Semir Zeki (1969) offered further evidence of the systematic
nature of the maps by showing that small lesions in V1 resulted in deterioration
of cells in corresponding parts of V2 and V3. In 1971 he repeated the approach
by making lesions in V2 and V3 and tracing their effects into areas on
the anterior bank of the lunate sulcus which he labeled V4 and V4a. Turning
to cell recording, Zeki established that cells in V4 responded to the wave
length of stimuli, while cells on the posterior bank of the superior temporal
sulcus (an area he labeled V5 but others have designated MT) responded
to motions of stimuli in specific directions (Zeki, 1973, 1974).

Various research from the 1950s to the early 1970s identified specific
responses to visual stimuli in areas of the temporal cortex and of the
posterior parietal cortex. Within the former, areas TE and TEO in the inferotemporal
cortex responded to specific shapes (Gross, Rocha-Miranda, & Bender,
1972). In posterior parietal cortex cells responded differentially to the
locations of stimuli (Goldberg & Robinson, 1980). In a relatively brief
period, such research defeated the hypothesis identifying visual processing
exclusively with processing in V1. Rather than undercutting the strategy
of hypothesizing identities, though, determining visual function in these
other areas led to more identity claims that were even more detailed and
that identified various aspects of visual processing with neural processes
in additional brain areas.

In 1982 Mishkin and Ungerleider proposed that visual processing in cortex
followed two pathways beyond V1, a ventral pathway into inferior temporal
cortex, which processed information about the identity of stimuli, and
a dorsal pathway into parietal cortex that processed information about
the location of stimuli (figure 8). Livingstone and Hubel (1984) extended
this proposal back to the retina. Some (Milner & Goodale, 1995) have
challenged the precise characterization of the processing in the two pathways,
but most research (van Essen & Gallant, 1994) supports the general
conception of two partially segregated processing streams (figure 9).

Figure 8: Mishkin and Ungerleider's (1982) representation of two visual
pathways in the brain

Figure 9: van Essen and Gallant's (1994) representation of the major
brain areas in the two visual pathways and their interconnections.

The discovery of multiple brain areas that seem to be processing different
visual information has proved the principal guide to detailed characterization
of visual processing in the brain, not top-down analyses in psychology
or artificial intelligence (e.g., motivated by Marr, 1982). Subsequently,
though, these neurobiological accounts have played a heuristic role in
developing higher-level analyses of vision. Ulric Neisser (1989) was one
of the first cognitive theorists to draw upon the two pathway account proposed
by Mishkin and Ungerleider. Neisser construed the dorsal pathway as embodying
Gibson's notion that we directly see the layout of the environment and
the ventral pathway as responsible for more inferential cognitive processing
(see also Milner and Goodale, 1995). Also, in the 1980s two groups of connectionist
modelers developed modularized networks that separately performed what
and where analyses of images on a simple retina in order to determine
the computational advantages of separate pathways (Jacobs, Jordan, &
Barto, 1991; Rueckl, Cave, & Kosslyn, 1989). Finally, psychologists
have recently developed behavioral measurers (e.g., speed of processing)
capable of demonstrating the difference in pathways in normal behaving
humans (Hale, 1996).

Conclusion

HIT (Heuristic Identity Theory) proposes that identity claims between
psychological processes and neural mechanisms are advanced as heuristics
that serve to guide further research. Emphasizing the thoroughly hypothetical
character of identity claims in science, HIT focusses on the way that proposed
identifications of psychological and neural processes and structures contribute
to the integration and improvement of our neurobiological and psychological
knowledge. Hypothesized identities advance research by suggesting new avenues
for the empirical investigation of both mind and brain. The resulting empirical
findings motivate scientists at both levels to tinker with their conceptions
of the pertinent processes and structures. As even the brief discussion
of visual processing demonstrates, these hypothetical identities evolve
in response to on-going research. Explanatory and predictive successes
are what justify these identity claim and what make additional theoretical
and evidential resources available in future research.

In response to both the correlation objection and the multiple realizability
objection, HIT stresses the importance of attending to the contributions
psychophysical identity claims have made over time to progressive programs
of research in neuroscience and psychology. It is difficult to imagine
that at the turn of the millennium any philosophers would regard these
considerations as even secondary, let alone irrelevant, to evaluating the
identity theory. We can think of no more reasonable grounds for adjudicating
these matters.

1. Although philosophers often speak of brain states,
neuroscientists are not generally interested in states but in brain areas
and brain processes.

2. There are many occasions when neurobiologists
employ a much finer grain. A major issue in recent years has been the plasticity
of cortex, often demonstrated by the rewiring of sensory processing areas
that occurs in response to altered sensory input. Even in the context of
comparative studies, there are times when neurobiologists are concerned
with micro-details (e.g., in measuring allometry or analyzing how connectivity
changes between species). When neurobiologists move to this grain size
for brain areas, though, they usually change the grain-size of their behavioral
measures as well and attend to differences in behavior between animals
or across species.